1. Field of the Invention
This invention relates to a method and system for continuously measuring the efficiency of drilling fluid. More specifically, the invention relates to a system and method for detecting cuttings accumulation and washout in wellbore during drilling operations by analyzing the return fluid containing drilling mud and cuttings exiting the wellbore.
2. Description of the Related Art
Drilling fluids are employed when drilling boreholes into subterranean formations. The drilling fluid xe2x80x9cmudxe2x80x9d consists of mixture of liquids and solids to provide special properties to better perform several primary functions in a drilling well. Drilling fluids lift the formation cuttings to the surface, control subsurface pressure, lubricate the drill string and bit, aid bottom-hole cleaning, aid formation evaluation, and provide protection to formation productivity.
One of the primary functions of the drilling fluid is the control of the formation pressure. The hydrostatic pressure exerted by the mud column, which is controlled by the density of the drilling fluid, is maintained above the pressure of the formation. If the formation pressure exceeds the pressure exerted by the mud column, formation fluid may enter the wellbore, causing a kick, which is any unscheduled entry of formation fluid into the wellbore. This results in a gain in the flow rate of the returning fluid. Additionally, the drilling fluid may incur losses due to the presence of a fracture in the formation. Fractures can result in loss of the drilling fluid, which results in a loss of the fluid flow rate at the surface. It is important to continuously monitor for the pressure of kicks and the fracture during drilling of wellbores. There are several methods and systems well known in the art that measure flow rate directly with various sensors.
Another primary purpose of the drilling fluid is to lift cuttings from the wellbore. The drilling mud is circulated down the drill string, through the bit, and returns to the surface through the annular space between the drill string and the wellbore wall. The mud returning to the surface is known as return fluid comprising drilling mud, formation particles called cuttings, and possibly some formation fluids. The drilled cuttings are picked up at the bit and returned to the surface for separation from the mud and for disposal. This removal of the drilled solids from the mud stream is critical to the subsequent reconditioning of the mud for recirculation in the well.
To control and improve drilling performance, evaluation of wellbore condition is important. Keeping the hole clean, especially in extended reach wells, is a key issue as cuttings accumulation in the annulus can contribute to, if not directly cause, pipe sticking and twist-offs. This is a concern when drilling a deviated well since a bed of cuttings is almost always formed on the lower side of the drill pipe. By measuring the cuttings discharge at the surface, the buildup of cuttings in the well can be detected early and remedial action taken to prevent a catastrophic failure.
Another obstacle encountered in drilling operations is washout. Washout is excessive borehole enlargement caused by solvent and erosion action by the drilling fluid. Washout can cause severe damage to the formation, contaminate the connate formation fluids, and waste costly drilling mud. Early detection through the measurement of cuttings exiting the wellbore can also help the mitigation of this problem.
In typical cuttings evaluation, the cuttings from the well are discharged over one or more shale shaker screens to separate them from the drilling mud, and all cuttings coming from the shakers are weighed. With expected cuttings density known by the user, the expected volume of the cuttings is calculated and the volume removed is compared to the volume calculated. Thus hole-cleaning efficiency is evaluated.
Currently the main types of mud out weight sensors used are a strain gauge and suspended heavy weight system, systems using differential pressure plates in the mud pit, and radioactive source sensors. Some of the mud adheres to the cuttings and is carried over with the cuttings discharged from the shale shaker. This portion of mud is lost to the mud system, which has been reported to be as high as two barrels of mud for every barrel of cuttings. The mud lost in the cuttings causes accuracy problems with the first two sensor types. The third system, although more accurate, is costly and requires certification and approval. The first two systems are not accurate enough for the cuttings removal performance application because of the settlement of the cuttings in the pits.
An object of the present invention is to provide an accurate, simple arid robust sensor system to evaluate hole cleaning performance. Two highly accurate pressure sensors are installed vertically displaced in a well riser to sense the pressure exerted by the return fluid including drilling mud and cuttings. Another object is to provide a processor for receiving signals from the sensors and for processing the data to determine hole cleaning performance. The advantage in measuring the return fluid is that the flow out including the cuttings is homogenous in the riser and no settlements occur.
In another embodiment, two additional sensors are provided to measure the drilling mud as it enters the well. With two sensors measuring return fluid pressure and two sensors measuring the pressure of drilling fluid entering the well, a processor can calculate efficiency based on more measured parameters. The processed data is an indication of well cleaning efficiency that can allow for early detection of washout or cuttings accumulation.
In another embodiment, sensors provided in a riser during tripping operations are used in conjunction with other sensors, such as flow rate sensors, to detect washouts and kick through the measurement of mud parameters entering the wellbore.